1. Technical Field
The present invention relates to the field of wireless communication, and more particularly, to a system architecture for management and coordination of multiple distributed remote radio units.
2. Discussion of Related Art
There is an ever growing need for faster communication with larger capacity, especially regarding cellular communication in crowded areas. One example is the distributed antennas system (DAS).
DAS system may be passive or active. Passive DAS uses passive components to distribute the RF signal. These passive components are coax cable, splitters, terminators, attenuators, circulators, couplers and filters (duplexer, diplexer or triplexer). Planning DAS includes calculating the maximum loss from base station to each antenna in the systems and the link budget for the particular area that each antenna covers. The passive DAS design needs to adapt to the limitation of the building regarding the restriction to where and how the heavy coax cable can be installed. A detail site survey of the building needed to be done to make sure that there are cable routes to all antennas.
Active DAS consists of a master unit (MU) connected to multiple expansion units (EU) with optical fiber up to 6 km in length. Each EU in turns connects to multiple remote units (RU) with thin coax or CAT5 cable up to 400 m in length. The MU controls and monitors the performance of the DAS. The UEs are distributed throughout the building and the RUs are installed close to the antenna. Active DAS has the ability to compensate for the losses of the cables interconnecting the components in the system by using internal calibrating signals and amplifiers. It does not matter what the distance between the antenna and the base station, all antennas in an active DAS will have the same performance (same noise figure and downlink power).
Both active and passive DAS suffer from several disadvantages. On the UL side the SNR is sensitivity limited due to the contribution of noise level from each RU reception signal. Furthermore, the brute force combining of all the RUs, could add interferences from RUs that don't receive any UE and “contribute” only interferers. On the DL side, the same signal is transmitted via all the RU's although it could contain irrelevant traffic for other spotted areas. That would cause DL interference for the macro deployment, neighbor small cells and redundant use of the radio resources. The DAS being RAN agnostic create a situation where the UL/DL signals couldn't be dynamically coordinated with respect to the RU. It would be agnostic to dynamic of the network traffic.
Another indoor solution is the small cell approach, deploying small IP-based cells as compact, standalone base stations with an integrated radio, baseband, and antenna unit. Base stations typically have integrated antennas, but sometimes antennas come separately. Femtocells can sit on desks, or mount on walls. Generally, picocells and femtocells connect to an IP Ethernet cable as backhaul or, in some cases, receive power over Ethernet. The small cell solution is based on deployment of Femto Access Points (FAP) or Pico cell that coordinated by SON management entity. The FAPs are connected to the Femto-GW via ethernet cables and the Femto-GW that concentrate all the FAPs is connected to the core network entities (SGSN and MCS). Small cells mainly come to provide capacity solution, but it has some challenges and limitations. Since it is most likely that the femto deployment would be in reuse, the system suffers from UL and DL interference between FAP's. Furthermore, the small cell deployment suffers from ping pong handovers due to the multitude of cells within a relevant small area. As a result it harms the QoE in mobility scenarios. Another disadvantage is the lack of effective utilization with regard to number of supported users, meaning that each small cell has a fixed maximum number of supported users (typically up to 32 active users) and as a result it could not support “hot zone” scenarios where many users are located near a single FAP.
There are systems which are using a coordinated small cell solution based on a local controller. The local controller unifies all small cells within each cluster and provides overall traffic aggregation and mobile session management for all services delivered through the cluster. Although it possess a coordination element the coordination inputs are limited to the information provided by L3 and the outputs are limited to the small cell flexibility, for instance it couldn't perform weight and combine, since this is a manipulation on the radio level.